Lecture 22: Geometry of Coordination Complexes

Read sections 19.7, 19.8 in your textbook.

In lecture 6 we discussed how to determine the shape of inorganic molecules using the VSEPR model. This works well for main group elements but it is less useful for the transition elements. Transition metals will seek to add ligands to attain an 18 electron count. Complexes with an 18 electron count are coordinatively saturated. The geometry depends greatly on steric factors (repulsion between coordinated ligands) rather than electronic factors repulsion between electron pairs).

Ligands coordinate metals through donor-acceptor bonds or through covalent bonds. When the bond is between one atom on the ligand and the metal, the ligand is monodentate. Some ligands coordinate to a metal with bonds from multiple atoms on the ligand. These ligands are multidentate (bidentate, tridentate, ...) or chelating ligands.

Cobalt has two bidentate or chelating ligands (glycolate and ethylene diamine) along with the monodentate ammonia ligands. The macrocyclic iron complex has only a tetradentate ligand.

Isomers are different molecules with the same molecular formular. They typically have different properties. Isomerization is particularly important with transition metal complexes because they can have multiple coordination environments and their ligands can exchange with others in solution.

Ionization isomers

Ionization isomers have different anionic groups as ligands and counterions but the same overall formula.

₃

₅

₄

₃

₅

₄

Hydration isomers

Transition metal complexes in aqueous solution have a first coordination sphere (the ligands, or the Lewis bases that coordinate directly to the metal) and a second coordination sphere (water molecules that are weakly bonded to the complex. When some of the water molecules from the second coordination sphere exchange with the ligands in the first coordination sphere hydration isomers result.

Coordination isomers

Coordination isomers can exist when the anion and the cation of a salt are metal complexes.

Linkage isomers

When a ligand has more than one Lewis basic site, it can coordinate to metals to form linkage isomers.

Geometric isomers

We've already covered the main types of geometric isomers in Lecture 6

cis, trans isomers

In a cis isomer, two of the same type of ligand are adjacent to one another in either an octahedral complex or a square planar complex. In a trans isomer, the two ligands are 180 degrees apart.
mer, fac isomers Meridinal isomers are three of the same type of ligand that forms a belt around an octahedral complex. The three ligands cover a triangular face in a facial isomer.

Optical isomers

Optical isomers are different only in their "handedness". They are not superimposible on their mirror images but are alike in all other respects. As in organic molecules, a tetrahedral complex with 4 different ligands is chiral. There is also an important class of tris chelate complexes that are chiral.

tetradedral complexes
"pseudotetrahedral" complexes
tris-chelates